Devices and methods for quartz enhanced photoacoustic spectroscopy

ABSTRACT

In quartz-enhanced photoacoustic spectroscopy (QEPAS), an analyte (typically in gas phase) generates a pressure wave in response to incident laser light. A quartz tuning fork (QTF) resonant at the frequency of the pressure wave transduces the pressure wave into an electrical signal. Pulsing the laser briefly reduces the amount of thermal chirp and increases the fraction of time that the laser emits at the wavelength(s) of interest. This increases the measurement efficiency. Pulsing the incident laser light with bursts of short pulses at the QTF resonant frequency increases signal strength. Exciting the sample with a two pulses at different laser wavelengths, separated by a half QTF period yields signal and background acoustic waves that partially cancel when integrated by the QTF, producing a differential measurement. Pulsing the incident laser light at a frequency faster than the gas response cut off frequency can improve the noise performance of a QEPAS measurement.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. application Ser. No.15/840,644, filed on Dec. 13, 2017, which claims the priority benefit,under 35 U.S.C. 119(e), of U.S. Application No. 62/433,487, filed onDec. 13, 2016. Each of these applications is incorporated herein byreference in its entirety.

BACKGROUND

Photoacoustic spectroscopy (PAS) is an analytical chemistry method thatrelies on the absorption of light by an analyte (typically in gasphase), whose subsequent collisional relaxation generates a pressurewave, detected by a microphone. Quartz-enhanced PAS (QEPAS) is aparticular implementation of PAS that uses an ultra-sensitive quartztuning fork (QTF) as the microphone. Typical modulation frequencies forQEPAS experiments are in the 5-100 kHz range, with 32.8 kHz being aparticularly favored frequency due to the widespread availability ofextremely cheap tuning forks resonant at this frequency, typically usedfor time measurement in wrist-watches.

In this case, the absorbed energy is transferred via a pressure wave toa mechanical resonator, typically in the shape of a tuning fork. Due toits fairly large quality factor (˜10,000), the mechanical resonatorintegrates the signal in the analog domain, before transduction into anelectrical signal, typically using the piezoelectric properties of thetuning fork material (e.g., quartz). This analog integration in themechanical domain reduces the readout noise of the electrical signal.

One advantage of QEPAS as a particular form of PAS is that no opticaldetector is required since the QTF acts as an uncooled, wavelengthinsensitive detector. Furthermore, PAS applications benefit from QTF'ssmall size (few mm³), extremely high quality factor (˜10,000 atatmospheric pressure), low cost (<$1 for a standard 32 kHz QTF), andexcellent repeatability. In addition, the noise level is ultimatelylimited by the thermal noise of the QTF, which is a few microvolts atroom temperature. The best results have been obtained in the mid-IRfingerprint range where the strongest target gas absorption lines occur.A record value of a few tens of parts-per-trillion (ppt) in volume wasobtained for sulfur hexafluoride (SF₆) detection employing an externalcavity mid-IR QCL fiber coupled to the QEPAS module.

SUMMARY

We present here several devices and methods aimed at improving QEPAS'ssignal strength and quality; its robustness against unwanted backgroundsignals of electrical, mechanical, and optical origin; and itsrobustness against varying environmental conditions (e.g., humidity,temperature, and the presence of different chemical species in the air).The devices and methods presented can be used to obtain quantitativemeasurements of the concentrations of target analytes. Furthermore,different implementations of QEPAS sensors are presented, includingQEPAS sensors whose sources and spectrophones are integrated for lowcost, easy assembly, and scalable production.

The description below uses QEPAS as an application field, although mostof the solutions presented are not specific to the use of a tuning forkas an acoustic detector and can thus apply to photoacoustic spectroscopyin general. In particular, the different modulation schemes, the use ofhumidity sensors, and the strategies for electrical pick-up noisecancellation are all applicable to general photoacoustic spectroscopy.

Embodiments of the present technology include methods of makingspectroscopic measurements of samples. Examples of these methods includemodulating a single-mode laser with a repetitive pulse sequence so as tocause the laser to emit a periodically pulsed laser beam. Each period ofthe repetitive pulse sequence comprises a plurality of pulses. Theperiodically pulsed laser beam illuminates the sample, which cases thesample to reflect, scatter, transmit, and/or emit radiation (e.g., lightor acoustic waves). A resonant detector whose resonance frequency isequal to a pulse repetition frequency of the periodically pulsed laserbeam detects this radiation.

In some implementations, the periodically pulsed laser beam has a dutycycle of less than 50%. Each period of the periodically pulsed laserbeam may comprises a burst of pulses, with each pulse in the burst ofpulses spanning a bandwidth of less than 1 cm⁻¹ or less than 0.2 cm⁻¹.In some cases, the burst of pulses spans less than half of the period ofthe periodically pulsed laser beam.

Each period of the periodically pulsed laser beam may comprise at leastone first pulse centered at a first wavelength and at least one secondpulse centered at a second wavelength different than the firstwavelength. The second pulse is delayed with respect to the first pulseby half the period of the periodically pulsed laser beam. The first andsecond pulses may cause the sample to reflect, scatter, transmit, and/oremit first and second radiation. If so, the resonant detector detectinterference between the first and second radiation.

Depending on the measurement, light reflected, scattered, and/ortransmitted by the sample may excite the resonant detector.Alternatively, the sample may emit an acoustic wave in response to theperiodically pulsed laser beam with the resonant detector. The resonantdetector detects this acoustic wave, which causes the resonant detectorto oscillate. These oscillations are converted into electrical signals,e.g., using the piezoelectric effect.

Another example method of making a spectroscopic measurement of a sampleincludes illuminating a sample with a periodically pulsed laser beam,each period of which comprises at least one first pulse centered at afirst wavelength and at least one second pulse centered at a secondwavelength different than the first wavelength. Again, the second pulseis delayed with respect the at least one first pulse by half the periodof the periodically pulsed laser beam. If desired, the temperature ofthe single-mode laser may be tuned between the first and second pulsesso as to change the output wavelength.

A resonant detector with a resonance frequency substantially equal to apulse repetition frequency of the periodically pulsed laser beamdetecting interference between first radiation reflected, scattered,transmitted, and/or emitted by the sample in response to the first pulseand second radiation reflected, scattered, transmitted, and/or emittedby the sample in response to the second pulse. Depending on themeasurement, the first radiation may represent an absorption resonanceof the sample and the second radiation may represent backgroundabsorption of the sample.

Yet another method of making a spectroscopic measurement of a sampleincludes emitting a sequence of pulses from a single-mode laser. Thesequence of pulses illuminates the sample, causing the sample totransmit, reflect, emit, and/or scatter radiation. A detection systemwhose a low-pass cutoff frequency is less than a pulse repetitionfrequency of the sequence of pulses detects this radiation.

Still another a method of making a spectroscopic measurement of a sampleincludes modulating a single-mode laser with a repetitive pulse sequenceso as to cause the laser to emit a periodically pulsed laser beam.Again, each period of the repetitive pulse sequence comprises aplurality of pulses. The periodically pulsed laser beam illuminates thesample, which reflects, scatters, transmits, and/or emit radiation(e.g., light or acoustic waves) in response. A detector generates anelectrical signal representing this radiation. And a filter, lock-inamplifier, processor, or other circuitry bandpass filters the electricalsignal at a band centered on the pulse repetition frequency.

It should be appreciated that all combinations of the foregoing conceptsand additional concepts discussed in greater detail below (provided suchconcepts are not mutually inconsistent) are contemplated as being partof the inventive subject matter disclosed herein. In particular, allcombinations of claimed subject matter appearing at the end of thisdisclosure are contemplated as being part of the inventive subjectmatter disclosed herein. It should also be appreciated that terminologyexplicitly employed herein that also may appear in any disclosureincorporated by reference should be accorded a meaning most consistentwith the particular concepts disclosed herein.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The skilled artisan will understand that the drawings primarily are forillustrative purposes and are not intended to limit the scope of theinventive subject matter described herein. The drawings are notnecessarily to scale; in some instances, various aspects of theinventive subject matter disclosed herein may be shown exaggerated orenlarged in the drawings to facilitate an understanding of differentfeatures. In the drawings, like reference characters generally refer tolike features (e.g., functionally similar and/or structurally similarelements).

FIG. 1A is a schematic of a system for quartz-enhanced photoacousticspectroscopy (QEPAS). The quartz tuning fork detector is typicallyinside the gas chamber since that is where the acoustic wave excitingthe tuning fork is usually generated. A humidity sensor is added in thegas chamber.

FIG. 1B is a schematic of a typical system for absorption spectroscopy.The detector is in this case typically outside of the gas cell.

FIG. 1C illustrates the different stages of signal generation for aphotoacoustic spectroscopy experiment, from the laser driver waveform tothe final detector output. This case represents the typical case wherebya single pulse is used to excite the acoustic wave, which in turnexcites mechanical vibrations of a mechanical resonator in the detector.

FIG. 1D illustrates the different stages of signal generation for aphotoacoustic spectroscopy experiment using a complex pulse pattern togenerate the acoustic wave, which in turn excites mechanical vibrationsof a mechanical resonator in the detector.

illustrates a complex but periodic pulse sequence for modulating asingle-mode laser (lower trace) and the acoustic modulation period(upper trace) generated by illuminating a sample with the laser lightemitted by the single-mode laser in response to the periodic pulsesequence.

FIG. 2A shows a simple pulse pattern with a single pulse per period formodulating a single-mode laser, such as a distributed feedback (DFB)quantum cascade laser (QCL).

FIG. 2B is a plot of the time evolution of the laser light frequencyemitted by a single-mode laser in response to the simple pulse patternshown in FIG. 2A, within one repetition period at 32.8 kHz, assuming asingle drive pulse of 300 ns starting at t=0, and a thermal relaxationtime constant for the active region of 0.8 μs (this value will be usedin all following examples).

FIG. 2C is a close-up of the plot in FIG. 2B showing with shadingindicating that if, for example, a gas only absorbs at laser wavelengthsbetween 2221.42222.6 cm⁻¹ and 22212222.8 cm⁻¹, then the photoacousticsignal is only generated during <50 ns of the total 300 ns of the pulse.

FIG. 3A illustrates a pulse pattern of 6 pulses each 50 ns in durationwith a 1 μs delay between pulses for modulating a single-mode laser.

FIG. 3B illustrates temperature fluctuations in a single-mode lasermodulated with the pulse pattern of FIG. 3A.

FIG. 3C illustrates wavenumbers of the laser output corresponding to thedifferent temperatures shown in FIG. 3B.

FIG. 3D illustrates a pulse pattern of 15 pulses each 20 ns in durationwith a 1 μs delay between pulses for modulating a single-mode laser.

FIG. 3E illustrates temperature fluctuations in a single-mode lasermodulated with the pulse pattern of FIG. 3D.

FIG. 3F illustrates wavenumbers of the laser output corresponding to thedifferent temperatures shown in FIG. 3E.

FIG. 4A is a plot of experimental emission spectra obtained for adistributed feedback QCL driven with different pulse patterns. The biasvoltage is kept constant. the repetition rate of the modulation patternis 32 kHz, and the sub-pulses are all grouped into the first 15 μs ofthe modulation period (˜30 μs).

FIG. 4B is a plot of measured increase in average power with increasingnumber of sub-pulses (crosses). The line is a linear interpolation.

FIG. 5 illustrates an example of a typical absorption spectrum showingon-line absorption band and off-line baseline.

FIG. 6A is a schematic illustrating a pulse pattern for a differentialspectroscopic measurement using a single-mode laser. During the firsthalf of the period (0<t<15.2 μs), a train of 14 pulses, each 20 ns induration and with a 1 μs delay between pulses. Then a second train ofsimilar pulses is fired in the second half, summed in amplitude to asingle 15 μs long pulse with a tenth of the amplitude of the previouspulses.

FIG. 6B is a plot of the time evolution of the laser active regiontemperature in response to the pulse pattern shown in FIG. 6A.

FIG. 6C is a plot of the time evolution of the laser emission wavenumberin response to the pulse pattern shown in FIG. 6A.

FIG. 7A is a schematic illustrating a pulse pattern for modulating asingle-mode laser to make a differential spectroscopic measurement(baseline subtraction). The modulation is as follows: during the firsthalf of each period (0<t<15.2 μs) of the 32.8 kHz periodic pattern, atrain of 14 pulses, each 20 ns in duration and with a 1 us delay betweenpulses is used, followed by a train of 5 pulses, each 60 ns in durationand with a 0.8 μs delay between pulses, during the second half period.All pulses have the same amplitude.

FIG. 7B shows expected temperature fluctuations of the laser's activeregion in response to the pulse pattern in FIG. 7A.

FIG. 7C shows wavenumbers corresponding to the different temperatures inFIG. 7B.

FIG. 7D shows a few cycles of the modulation of FIG. 7A, repeated with a30 μs period.

FIG. 8A is a schematic illustrating another pulse pattern for adifferential measurement (baseline subtraction). The modulation is asfollow: during the first half of each period (0<t<15.2 μs) of the 32.8kHz periodic pattern, a train of 50 pulses, each 20 ns in duration andwith a 280 ns delay between pulses is used, followed by a train of 50pulses, each 30 ns in duration and with a 270 ns delay between pulses,during the second half period.

FIG. 8B shows expected temperature fluctuations of the laser's activeregion in response to the pulse pattern in FIG. 8A.

FIG. 8C shows wavenumbers corresponding to the different temperatures inFIG. 8B.

FIG. 8D shows a few cycles of the modulation of FIG. 8A, repeated with a30 μs period.

FIG. 9A shows a simulation of an acoustic waveform excited by theabsorption of a laser modulated with a single 300 ns long pulse perperiod and a pulse repetition rate of 32.8 kHz modulation, assuming avibrational-translational relaxation time constant for the gas analyteof 30 μs.

FIG. 9B shows an acoustic waveform obtained with a laser modulated witha train of 15 pulses, each 60 ns in duration and fired during the firsthalf of the modulation period for a 32.8 kHz pulse repetition rate

FIGS. 10A-10C illustrate how a three-prong tuning fork generates anacoustic wave in response excitation by a laser in quartz-enhancedphotoacoustic spectroscopy (QEPAS).

FIG. 11 is a schematic of a QEPAS setup using two tuning forks wired insuch a way as to subtract the electrical signals generated by eachtuning fork.

FIG. 12 is a schematic of circuitry that performs electromagneticpick-up noise subtraction. A switch, realized, for example, withtransistors directly defined on the semiconductor laser chip (e.g.,indium phosphide in the case of quantum cascade lasers), can allow forthe alternative drive of the laser source and of a dummy load withsimilar electrical characteristic than the laser, located in closeproximity to the laser, on the same chip.

FIG. 13 is a schematic of a QEPAS device that can use the pulse patternsshown in the preceding figures.

FIG. 14 is a schematic showing the use of an aperture for spatialfiltering with a tuning fork for a QEPAS measurement.

FIGS. 15A-15I illustrate geometries for QEPAS devices based onedge-emitting semiconductor lasers. All schematics show the device seenfrom the top. The laser epi layers are in the plane of the schematic.The tuning fork prongs are represented as two crossed rectangles.

FIGS. 16A and 16B are schematics showing the use of on-chip opticalelements to image the laser facet onto a point located between thetuning fork prongs.

FIGS. 17A-17C show different QEPAS device configurations usingsurface-emitting lasers.

FIGS. 18A and 18B show side and front views, respectively, of a laserarchitecture with a substrate emitting laser.

FIG. 19 shows an exploded view of an integrated QEPAS sensor(subassembly) with a laser source, optical element, aperture and tuningfork.

FIG. 20 illustrates a wafer-bonding process suitable for making anintegrated QEPAS sensor like the one shown in FIG. 20, whereby thedifferent functionalities are merged into a single subassembly bybonding wafers with two-dimensional arrays of lasers, lenslet arrays,apertures, and tuning forks.

FIGS. 21A-21D show a set of geometries based on surface-emittingsemiconductor lasers. All schematics show the device seen from the side(cross section along the length of the lasers). The tuning fork prongsare represented as two crossed rectangles in FIGS. 21B-21D.

DETAILED DESCRIPTION

The devices and methods presented here address issues related to the useof semiconductor lasers as a source of light for a PAS spectrometer. Forexample, when using a pulsed distributed-feedback (DFB) quantum cascadelaser (QCL) emitting in the infrared spectral range, the heating of theQCL's active region during the pulse duration (on the scale of a fewnanoseconds to a few microseconds) leads to a change of the refractiveindex and consequently a chirp of the emitted laser light (i.e., achange of frequency versus time). For a pulse duration on the order of afew hundred nanoseconds, that chirp is typically on the order of 1 to 3cm⁻¹, although this value can be made smaller or larger based on theheat dissipated in the QCL's active region. For example, a laser withhigh doping of the active region typically draws more current and thusdissipates more heat, leading to a larger chirp rate. The efficiency ofthe active region and thus the amount of heat it generates for a giveninput electrical power can also be adjusted by proper band-structureengineering.

Chirping due to temperature changes reduces the efficiency of a PASmeasurement because gas absorption lines, even at ambient conditions oftemperature and pressure, are typically narrower than the chirpbandwidth. To see why, consider a gas line with an absorption linewidthof Δv_(gas) (which may be smaller than 1 cm⁻¹) and a laser that chirpsover Δv_(chirp) during the duration of the electrical driver pulse τ,where we assume that Δv_(chirp)>Δv_(gas) and that Δv_(chirp) encompassesthe full absorption linewidth. The effective time during which the laserlight is absorbed by the analyte is thus on the order of(Δv_(gas)/Δv_(chirp))τ. In other words, only a fractionΔv_(gas)/Δv_(chirp) of the laser energy contributes to the generation ofthe photoacoustic signal, the remainder being wasted.

This observation is generalizable to other spectroscopic techniquesemploying slow detectors, in which an average quantity is measured. Aslow detector refers here to a detector with a time constant larger thanthe typical time it takes a pulsed laser to chirp through a gas line.For example, for infrared spectroscopy based on pulsed quantum cascadelasers, a detector with a time constant larger than 1 microsecond can beconsidered slow since such detector may not be able to resolve theabsorption dip as the laser chirps through a gas line during a singlepulse. Note that other elements of the system (e.g., the pre-amplifieror the digitizer) may limit the bandwidth and that the overall systemresponse speed should be considered. When a slow detector is used, thequantity measured is typically the total amount of light absorbed by thegas over the pulse duration, measured indirectly via the amplitude ofthe consequently generated acoustic wave for PAS, or directly as atransmission dip in more typical infrared absorption spectroscopy. Inthese cases, only a fraction Δv_(gas)/Δv_(chirp) of the laser energycontributes to the generation of information.

Another spectroscopic technique using a slow, resonant detector is theuse of a mechanical resonator as a light detector. A mechanicalresonator, for example, a quartz tuning fork, can be excited byphotothermal effects when the radiation impinging on the resonatormaterial is partially absorbed by the material, leading to a localheating of the material and consequent strain. The photoelectric effectmay also result in the creation of surface charges when the light isimpinging on the resonator material, which may in turn strain thematerial if the material has a significant piezoelectric coefficient.The techniques described here, aimed at increasing or maximizing signalstrength and mitigating background signal and noise in QEPAS, also applyto most techniques employing slow detectors, such as the resonantthermomechanical technique described in this paragraph.

High Frequency Modulation

It is usually assumed by QEPAS practitioners that the modulationfrequency should be similar or slower than the analytevibrational-translational relaxation time, which acts as a low-pass forthe photoacoustic signal. However, there are several advantages to usingmodulations faster than the analyte vibrational-translational relaxationtime:

(1) A fast modulation can be used to increase the effective duty cycleof the measurement. As mentioned above, long laser pulses lead to largerintra-pulse chirp range. When Δv_(chirp)>Δv_(gas), which is commonly thecase, increasing the laser pulse duration does not lead to an increasein signal since it increases the chirp range Δv_(chirp). In other words,the laser is not exciting the gas molecules during that extra duration.Increasing the modulation frequency (i.e., decreasing the pulserepletion period) at constant pulse width thus leads to a proportionalincrease of energy transferred to the gas per unit of time, resulting inan increase of acoustic signal generated per unit of time proportionallyto the modulation frequency.

(2) A fast modulation can help isolate the generated acoustic signalfrom ambient noise and 1/f noise. To see how a modulation faster thanthe analyte vibrational-translational relaxation time can be used,consider a measured photoacoustic signal S that is proportional toa(f)*P_(average)*Q, where a(f) is a factor representing the efficiencyof the acoustic wave generation for a modulation at frequency f,P_(average) is the laser average power (proportional to f if the pulsewidth is fixed), and Q is the fork quality factor. The effect of thelimited V-T relaxation time is similar to that of a first order low-passfilter, resulting in a coefficient a(f) approximately proportional to1/f Consequently, when increasing the modulation frequency fin anexperiment involving pulsed lasers with Δv_(chirp)>Δv_(gas), thephotoacoustic signal is approximately conserved. However, the fastfrequency operation can enable easier frequency-based filtering fromambient noise, 1/f noise, etc.

PAS with Pulsed Semiconductor Lasers

Quantum cascade lasers (QCLs) are often used as sources of light inQEPAS spectrometer since they can emit light in the mid-wave andlong-wave infrared regions (3-16 μm) and beyond in the terahertzspectral range. These spectral ranges are relevant for analyticalchemistry since many molecules have unique absorption features (such assharp absorption lines) corresponding to the excitation ofro-vibrational modes of the molecules. Measuring a spectrum containingthese features can allow for the identification and quantification ofthese molecules.

Most QEPAS experiments to date using QCLs use continuous wave (CW)devices that emit light continuously for at least the duration of themeasurement. In contrast to pulsed QCLs, CW-QCLs require good thermaldissipation, with design implications from the package to the laserwaveguide level, driving constraints for system weight, size, powerconsumption, manufacturing cost, and laser fabrication yield. Theselasers are also usually on the order of ten times less efficient atconverting electrical power into optical power than pulsed QCLs. It canthus be advantageous to use pulsed QCLs in situations that benefit fromsmall size, weight, power, and cost. Furthermore, since the frequencytuning obtained with the intra-pulse chirp of pulsed lasers is anintrinsic effect that does not require additional components or control,pulsed QCLs provide a faster, simpler and potentially more accuratesolution than the typical current or temperature ramps used in thefrequency tuning of CW lasers.

We present here processes that can be used to optimize the amount ofacoustic energy generated from the gas absorption of the laser light,and the amplitude of the signal readout from the piezo-electrictransducer. These processes include increasing the efficiency ofacoustic signal generation by increasing the time during which the laseris on and the frequency of its emitted light overlaps the gas absorptionline targeted by the measurement. They also include increasing thepiezoelectric signal by (1) modulating the laser light at or close tothe resonant frequency of the tuning fork; and/or (2) reducing theenergy that is lost to the excitation of higher harmonics of themechanical oscillator (unless these are also measured and contribute tothe acquired signal).

FIG. 1A shows a system for conducting QEPAS measurements using inventivearchitectures, inventive pulsed modulation schemes, or both. The systemof FIG. 1A includes a laser driver 101 that applies a repetitivesequence of electrical pulses to a single-mode laser 102, such as a DFBQCL. The single-mode laser 102 emits a repetitive sequence of opticalpulses in response to the repetitive sequence of electrical pulses. Anoptical system 103 comprising lenses, mirrors, prisms, and/or otheroptical components shape and direct these optical pulses to a gas cell104 that contains a resonant detector (e.g., a quartz tuning fork 107)and an optional humidity detector 112.

In operation, an analyte gas flows into the gas cell 104 via an inlet105 and out of the gas cell 104 via an outlet 106. The optical system103 focuses the optical pulses between the prongs of the tuning fork107, which may have a resonance frequency from about 1 kHz to severalMHz (e.g., 32 kHz resonance). The focused optical pulses heat theanalyte gas between the tuning fork prongs, which drives a pressure wave(i.e., an acoustic wave), which in turn excites the tuning fork 107.Alternatively, the optical system 103 may focus the optical pulses ontothe tuning fork 107 for resonant thermomechanical spectroscopymeasurements, which involve exciting the tuning fork directly with thelight instead of with an acoustic wave in the gas analyte.

A pre-amplifier 108 and lock-in amplifier 109 read the tuning fork'smechanical oscillations as an electrical signal via the piezoelectriceffect. (Alternatively, the tuning fork oscillations can be readoptically using an interferometer or deflectometer.) Ananalog-to-digital converter (ADC) 110 digitizes the output of thelock-in amplifier 109 for processing by a computer 111, which uses theelectrical signal and the humidity sensor's output to control the laserdriver 101. Lock-in detection may also be realized in digital space: inthis case, the output of the pre-amplifier is first digitized, andalgorithms (e.g. Fourier transforms and digital filtering) may be usedto extract the signal amplitude and phase at a specific frequency (e.g.the resonance frequency of the resonant detector).

FIG. 1B shows an inventive system for absorption spectroscopy. Itincludes all of the same components shown in FIG. 1A, except that thedetector 107 is outside of the gas cell 104. The detector in theabsorption spectroscopy system of FIG. 1B can be either a resonantdetector or a non-resonant detector. For instance, the detector may be aslow detector as described above. If desired, the lock-in amplifier 109can bandpass filter the electrical signal (e.g., current or voltage)generated by the detector 107 at a band centered on the pulse repetitionfrequency of the electrical signal used to modulate the laser 102. (Thisbandpass filtering may also be accomplished with a separate analogfilter or in the digital domain, e.g., with a processor.) This reduces1/f noise and make it possible to deposit more energy in a narrowwavelength bandwidth by emitting many short optical pulses from thelaser 102.

FIG. 1C shows the inputs and outputs in a conventional QEPAS orabsorption spectroscopy measurement using a resonant detector. The toptrace shows the laser drive voltage, which is a pulsed modulation schemewith a single electrical pulse per repetition period. The next traceshows the laser output power, which is also a single optical pulse perrepetition period. When focused to a point in the gas, these opticalpulses produce acoustic waves with sharp leading edges and exponentiallydecaying trailing edges as shown in the middle trace. The acoustic wavesexcite quasi-sinusoidal mechanical oscillations in the resonant detector(tuning fork) that in turn produce a quasi-sinusoidal electrical signalas shown in the bottom two traces of FIG. 1C.

Instead of this usual method, we modulate a single-mode laser, such as aDFB QCL, with a complex pulse pattern formed by the periodic repetition(for example, at the tuning fork resonance frequency) of a shortsequence of pulses, with varying duration, amplitude, and delays. Thetop trace in FIG. 1D shows one example of this complex pulse pattern.The single-mode laser emits the complex optical pulse pattern shown inthe next trace in FIG. 1D (there may not be a one-to-one correspondencebetween the electrical pulses modulating the laser and the opticalpulses emitted by the laser). In a QEPAS measurement, these opticalpulses produce sawtooth-like acoustic waves (middle trace in FIG. 1D),which cause sine-like mechanical oscillations of the tuning fork(second-lower trace in FIG. 1D that in turn yield sine-like electricalsignals (bottom trace in FIG. 1D).

We call the repetition period of the complex pulse pattern the “acousticmodulation period”, as shown in FIG. 1D, to emphasize the fact that itis the period at which the acoustic waves are generated in order toexcite the resonant detector used to sense the laser light reflected,transmitted, and/or scattered by the sample. It corresponds to thefrequency that one may want to match to the resonant frequency of theresonant detector, which may be a tuning fork or an acoustic resonator.

The number of pulses composing this repeated pattern used to modulatethe laser is another design parameter. By synthesizing this complexpulse sequence, it is possible to (1) shape the temperature profile(over time, within each modulation period) of the active region and thusthe laser light frequency variation over time, within the duration ofthe modulation period; and (2) shape the frequency spectrum of thegenerated acoustic wave, and in particular the energy distributionbetween the fundamental modulation and its harmonics. In doing so, it ispossible to increase the energy transfer to the acoustic wave and thestrength of the signal measured by the resonant detector. Severalstrategies can be employed, including but not limited to: adjusting thenumber of sub-pulses, their heights, and their durations; adding subthreshold pulses to rapidly tune the laser center frequency; and addinga long and low amplitude pulse to the train of sub-pulses.

If desired, the pulse sequence shown in the top trace of FIG. 1D may bevaried to slowly ramp the center frequency of the optical pulses emittedby the laser. For example, the pulse sequence may include a constantnumber of pulses per sequence period (e.g., 20 pulses in 15 μs, repeatedevery 30 μs), but with a slowly decreasing delay between pulses (e.g.,from 0.79 μs to 0.3 μs). If desired, the pulse duration may vary fromabout 5 ns to about 5 μs. And the please pulse power may range fromabout 10 mW to about 500 mW. The exact pulse sequence, duration, delay,and amplitude may depend on the laser geometry, laser doping, how muchenergy is deposited in the active region volume, and what is the thermalconductivity of this volume. But as described below, a steady state canbe established in just a few cycles (e.g., <5 μs total time), so thateven if the temperature of the active region does not relax fully inbetween pulses, the temperature range spanned by a pulse can be equal tothe one spanned by the previous pulses.

FIGS. 2A-2C show the time evolution of the modulation and laser lightfrequency for a single-mode pulsed laser. FIG. 2A shows the modulationused to drive the laser. The modulation has a pulse repetition frequencyof 32.8 kHz, which matches the resonance frequency of readily availablequartz tuning forks, and include a single drive pulse of 300 ns startingat t=0. FIGS. 2B and 2C show the wavelength of the laser, which has athermal relaxation time constant for the active region of 0.8 μs (thisthermal relaxation time constant is used in all following examples). Itshould be noted that the laser is only emitting light during thedownward slope of the curve, while the upward slope reflects the coolingof the active region after the electrical driving pulse terminates. Ascan be seen, the laser chirps over 1.3 m⁻¹ during the short duration ofthe electrical pulse, which is significantly larger than most gasabsorption lines. A zoom-in in FIG. 2C shows that if, for example, a gasonly absorbs between 2222.6 cm⁻¹ and 2222.8 cm⁻¹, then the photoacousticsignal is only generated during <50 ns of the total 300 ns of the pulse.

FIGS. 3A-3F show similar data for a laser modulated with a 300 nsdriving pulse split into 6 pulses each 50 ns in duration (FIGS. 3A-3C)or 15 pulses each 20 ns in duration (FIGS. 3D-3F), with a 1 μs delaybetween pulses in both cases. This corresponds to the same total ON timefor the laser in FIGS. 2A-2C, and thus the same energy consumption andtotal heat dissipation. However, by spreading that heat deposition overhalf the repetition period, the amplitude of the temperature variationof the active region, and consequently the amplitude of the chirp in theemitted laser beam is reduced. This enables more efficient use of thelaser energy to generate the acoustic signal, since the frequency of theemitted laser light overlaps the analyte absorption band for a longerfraction of the time it is turned on.

FIGS. 3A and 3D are schematics illustrating the pulse patterns used tomodulate the laser. FIGS. 3B and 3E show the expected temperaturefluctuations of the active region in response to the pulse patterns inFIGS. 3A and 3D, respectively. (Note that the exact temperaturevariations may depend on the laser design, and in particular the laserridge geometry, the active region doping, the active region efficiency,etc.) Finally, FIGS. 3C and 3F show the wavenumbers corresponding to thedifferent temperatures in FIGS. 3B and 3E, respectively: there is aone-to-one mapping between the laser temperature and the wavenumber ofthe emitted light. Note that the laser is only lasing during the pulses,i.e., during the portion of the curves where the temperature increasesand the wavenumber decreases. Note that the pulse height is indicatedhere and in the following figures as 15 V, as an example of abovethreshold voltage. This pulse height may be adjusted to the drivecondition of the particular laser used.

FIGS. 3A-3F illustrate the increase in average power emitted within theabsorption bandwidth of a narrow-line gas absorber, using the simulatedtemperature response of a QCL active region, and the correspondingwavenumber chirp of the laser light. As can be observed, breaking out asingle long pulse into multiple shorter pulses can reduce the wavenumberrange over which the laser chirps, thereby increasing the total powerdeposited within the narrow absorption bandwidth of a gas line.

FIG. 4A reports experimentally measured emission spectra confirming thiseffect using a pulsed distributed feedback quantum cascade laser. Itshows how an increase in power is obtained within a narrow spectralbandwidth (0.5 cm⁻¹), for a constant laser bias, for an increasingnumber of short, identical sub-pulses. Each trace represents theemission spectrum for a different number of sub-pulses in each period ofthe laser modulation signal, with the number of pulse ranging from 1(lowest peak) to 20 (highest peak). For this experiment, the repetitionrate of the modulation pattern was 32 kHz, and the sub-pulses were allgrouped into the first 15 μs of a quartz tuning fork modulation period(˜30 μs).

FIG. 4B is a plot of average power emission spectrum versus number ofsub-pulses (crosses) used to modulate single-mode power. The line is alinear interpolation. By allowing cooling of the active region betweenconsecutive short pulses, the efficiency of the laser is also increasedand we can observe in FIG. 4B that the average power scales linearlywith the number of sub-pulses. This is to be contrasted to a drivescheme with a single pulse of increasing duration, for which typicallythe increase in average power with pulse width is sub-linear, reflectingthe reduction in efficiency as the active region temperature increaseduring the pulse.

Differential PAS Measurements

Several QEPAS results reported in the literature use a so-called2f-demodulation scheme. In this method, a CW laser is modulated by asine wave such that its center frequency also follows a sine wavemodulation centered on the gas absorption line. The laser centerfrequency thus crosses the peak of the absorption line twice within aperiod. In other words, a strong acoustic signal is generated at twicethe frequency of the laser modulation. A lock-in set to detect thatsecond harmonic will thus reject any signal at the fundamental lasermodulation frequency, which could originate from a variety of sourcessuch as electrical pick-up, light absorption by the tuning fork if theoptical beam hits the fork, or acoustic signal generated by absorbers inthe beam path, such as optical windows. Note that for efficientexcitation of the second harmonic, the absorption line should besymmetric about its peak and the laser center frequency (i.e., its meanfrequency over a modulation period) should match the peak absorption.

The 2f-demodulation technique cannot be implemented with standard pulsedpatterns since the laser frequency crosses the maximum absorption onlyonce per modulation period. However, we present here an alternative withsome of the same advantages. For the sake of example, consider anabsorption spectrum as shown in FIG. 5, featuring a narrow absorptionline and a baseline absorption in the vicinity that is relativelyfeatureless. The position and width of the two bands (‘absorption line’and ‘baseline absorption’) are arbitrary here and in the followingfigures.

FIGS. 6A-6C illustrate a differential spectroscopic technique using apulsed single-mode laser, such as a DFB QCL. FIGS. 6B and 6C show thetemperature of the laser's active region and the laser wavenumber versustime, respectively, during one modulation period at 32.8 kHz for asingle-mode laser modulated with the modulation pattern shown in FIG.6A. During the first half of the period (0<t<15.2 μs), a train of 14pulses, each 20 ns in duration and with a 1 μs delay between pulses.Then a second train of similar pulses is fired in the second half,summed in amplitude to a single 15 μs long pulse with a tenth of theamplitude of the previous pulses. That long pulse allows for an offsetof the active region temperature and thus an offset in the wavenumber ofthe emitted light between the two half periods.

FIG. 6C shows the result of the modulation shown in FIG. 6A: (1)assuming the laser wavenumber overlaps the absorption line in only thefirst half period, the photoacoustic signal is only generated in thefirst half period, and it is modulated at 32.8 kHz, assumed to be herethe resonance frequency of the tuning fork; (2) a second out-of-phasephotoacoustic signal is generated during the second half of themodulation period. This signal is also modulated at 32.8 kHz.

By adjusting the amplitude, width or delay between the sub-pulses inFIG. 6A, it is possible to balance the two signals in FIG. 6C (on-lineand off-line), for example, in the absence of the target gas analyte(e.g., during ‘zero-gas’ calibration), to obtain a differentialmeasurement. Because the two signals are out-of-phase with respect tothe acoustic modulation period, they subtract from one another in thefinal measured signal; (3) Finally, a background absorption or signal(e.g., from optical window absorption, or from the laser light hittingthe tuning fork prongs) that is frequency insensitive on the scale ofthe intra-pulse laser wavenumber variations contributes to an equalsignal during both half periods. In other term, the correspondingphotoacoustic signal is modulated at 2f and thus can be rejected if theamplitude of only the fundamental modulation is measured. Note that thisis the opposite strategy of the usual 2f-demodulation scheme often usedin QEPAS, where the useful signal is modulated at 2f and the backgroundis modulated at f.

The modulation scheme shown in FIG. 6A has several advantages, includingbut not limited to:

-   -   1) If a differential measurement is realized this way, between        on- and off-line frequency bands, the measurement can have a        reduced sensitivity to laser power variations over time.    -   2) If a strong background signal is generated (for example, if        the laser beam hits the prong of a tuning fork), it can be        separated from the useful chemically relevant signal since the        background and chemically relevant signals are modulated at        different frequencies.    -   3) By adjusting the amplitude, delay and width of the pulse        sub-pattern, it is possible to adjust the amount of        photoacoustic signal generated in different sections of the        modulations. This can be used to increase the amount of        photoacoustic signal generated from the absorption of the target        analyte, reduce the amount of photoacoustic signal generated by        baseline or background absorption, or balance the generation of        photoacoustic in different sections of the modulations (for        example, in- and out-of-phase signal components).    -   4) Any potential electrical pick-up signal is mostly modulated        at a much higher frequency than the tuning fork resonance.        Therefore, the modulation scheme can decrease the background        signal (and its associated noise) due to electrical pick-up at        the frequency at which the chemical information is coded and        measured.    -   5) The advantages of this modulation scheme do not rely on a        symmetry of the absorption feature about its peak. It can be        implemented for fairly broad absorption features, assuming the        laser can be tuned far enough.

We assumed above a narrow absorption line such that the laser centerfrequency can be tuned from ‘on-line’ to ‘off-line’ on a time scalesmaller than the acoustic modulation period, by adjusting the complexpattern parameters (numbers of pulses, their amplitude, duration, delay,etc . . . ). Some gases have relatively broad absorption lines (fullwidth at half maximum of a few wavenumbers). For these gases, it may notbe practical or even possible to switch between on- and off-line withintra-pulse tuning, although in the case of quantum cascade lasers, highdoping of the active region can allow for intra-pulse tuning on thescale of 10 cm⁻¹ per microsecond at ˜4.5 microns wavelength. However,the modulation scheme presented here to achieve a differentialmeasurement can still be used to measure the derivative (i.e., theslope) of the absorption line. A slow temperature ramp of the submountor heat sink can be used to slowly (i.e., at a time scale larger thanthe acoustic modulation period) tune the laser center frequency over theabsorption line.

FIGS. 7A-7D illustrate another differential spectroscopic techniqueusing a pulsed single-mode laser. FIG. 7A is a schematic illustratingthe pulse patterns. FIG. 7B shows the expected temperature fluctuationsof the single-mode laser's active region in response to the pulsepattern shown in FIG. 7A. FIG. 7C shows the wavenumbers of the laseroutput corresponding to the different temperatures shown in FIG. 7B.FIG. 7D shows a few cycles of the periodically pulsed laser beam,repeated with a 30 μs period.

Since it can be difficult in practice to adjust the electrical drivingpulse height at the microsecond scale, or to sum two drive pulses as isused in FIGS. 6A-6C, we show in FIG. 7A another possible pulse patternobtained by adjusting only the pulse durations and delays. In FIG. 7A,the modulation is as follow: during the first half of each period(0<t<15.2 μs) of the 32.8 kHz periodic pattern, a train of 14 pulses,each 20 ns in duration and with a 1 μs delay between pulses is used,followed by a train of 5 pulses, each 60 ns in duration and with a 0.8μs delay between pulses, during the second half period. All pulses havethe same height.

We note that it may be desired to tune the pulse pattern so that theon-line and off-line signals have equal amplitudes in the absence ofanalyte. This ensures zero signal in the absence of analyte and azero-offset differential signal in the presence of an analyte. Forexample, one may adjust the number of pulses in each sub-sequence ortheir duration to achieve such tuning.

FIGS. 8A-8D illustrate yet another example of differential spectroscopicmeasurements using a pulsed single-mode laser. In FIG. 8A, themodulation of the laser is as follow: during the first half of eachperiod (0<t<15.2 us) of the 32.8 kHz periodic pattern, a train of 50pulses, each 20 ns in duration and with a 280 ns delay between pulses isused, followed by a train of 50 pulses, each 30 ns in duration and witha 270 ns delay between pulses, during the second half period. FIGS. 8Band 8C show the temperature of the laser's active region and thewavenumber of the laser's output, respectively, as a function of timewhen the laser is modulated with the pulse pattern shown on FIG. 8A. Afew periods of the 32.8 kHz modulation are shown in FIG. 8D. Note thatthis later pulse scheme has a higher duty cycle (8.2%), implying higherheat dissipation of the laser.

The pulses pattern presented here are examples to illustrate differentdegrees of freedoms that are available to the designer. In general, wepresent using short burst of pulses within each period of the overallperiodic signal (usually in resonance with the resonant detector, e.g.,a tuning fork). The number, timing (duration, delay), and amplitude ofthe pulses can be adjusted to shape the active region temperature versustime, and thus the emitted light frequency versus time, on a time scalesmaller than the acoustic modulation period. This is different fromusual modulation schemes that typically use a simple periodicmodulation, and sometimes add a slow modulation (for example, a drivecurrent ramp) to slowly scan the laser center frequency through anabsorption line at a time scale larger than the modulation period usedto retrieve the signal.

The photoacoustic signal generation is governed by the V-T relaxationtime of the molecules absorbing the light pulses. The V-T relaxationtime is typically on the scale of tenth to hundreds of microseconds. Bychanging the pulse sub-pattern within the periods of the modulation, itis possible to shape the acoustic signal generated and thus thedistribution of energy between the fundamental and the harmonics of themodulation, in order to increase or maximize the strength of themeasured signal, depending on the demodulation scheme used. For example,if a lock-in amplifier is used to measure the amplitude of thefundamental modulation frequency, it may be desirable to maximize thisamplitude by shaping the acoustic waveform. Alternatively, the amplitudeof a higher (n>2) harmonic may be measured, in which case it may bedesirable to increase the excitation of higher harmonics.

FIGS. 9A and 9B show two different acoustic waveforms obtained with twodifferent pulse schemes: in FIG. 9A, the modulation is a single 300 nslong pulse for each period of the 32.8 kHz modulation. In FIG. 9B, atrain of 15 pulses, 60 ns in duration each is fired during the firsthalf of the 32.8 kHz modulation period. As can be seen, the first schemeresults in a sawtooth acoustic waveform shape while the second schemeresults in a triangular waveform.

Integration of a Humidity Sensor

The photoacoustic signal strength is strongly impacted by the value ofthe V-T relaxation time of the analyte molecules. This time is itself afunction of the gas composition (in addition to temperature and pressuresensitivities). In particular, the presence of water molecules has beenshown to reduce the relaxation time and thus enable the generation ofstronger signals when a fast modulation is used.

Consider a photoacoustic gas sensor measuring ambient air, with varyinghumidity. An initial calibration can be realized by measuring the effectof water concentration on the photoacoustic signal strength, for varyinganalyte concentrations, thus establishing an empirical relationshipbetween water concentration and photoacoustic signal strength forvarying analyte concentration. For instance, this empirical relationshipmay be established by calibrating the sensor (i.e., measuring the signalversus analyte concentration) under different humidity conditions andusing the measurements to create calibration curves parametrized byhumidity.

Then, a quantitative measurement can be obtained by (1) integrating ahumidity sensor (or other sensor determining water vapor concentration)with the QEPAS sensor to measure the water concentration in the analyte;(2) measuring water concentration in the analyte at the time ofmeasurement and using the established empirical relationship to modelthe expected photoacoustic signal strength as a function of analyteconcentration; and (3) deriving the analyte concentration from themeasured signal and established empirical relationship between waterconcentration and photoacoustic signal strength.

Electrical Pick-Up Signal Cancelation

One source of noise observed in QEPAS experiments is associated withpossible electrical pick-up signal. For example, the laser driver,especially when driving pulsed lasers with short, high amplitudeelectrical pulses, can radiate electromagnetic waves that can bepicked-up by the electronic circuit between the fork and thepre-amplifier, via electromagnetic induction. Ground loops are alsooften responsible for the apparition of pick-up signals. This problemcan be exacerbated with lock-in detection, which will amplify any signalmodulated at the same frequency as the pulse repetition rate. Wepresented above how the modulation scheme can be varied to mitigate thiseffect by modulating such pick-up signal at 2f and higher harmonics,while the chemically relevant signal is modulated at f.

Embodiments of the present technology include devices where the lasersource is tightly integrated with the QEPAS fork and relatedelectronics. In such devices, electrical pick-up signals can be largerbecause of the difficulty in electromagnetically shielding separatecomponents from one another. Here, we present alternative devices andmethods to mitigate this issue.

FIGS. 10A-10C shows a three-prong tuning fork where all three prongs 1,2, and 3 are mechanically coupled (as illustrated by the dashed line inFIG. 10A). This tuning fork can be used as a resonant detector in anintegrated QEPAS sensor. The laser light emitted by a single-mode laserin the QEPAS sensor is transmitted, scattered, and/or reflected by thesample and passes in between two prongs 1 and 2 but not through theother pair of prongs 2 and 3, as shown in a top view of the fork in FIG.10A.

The wiring of the tuning fork is such that a voltage V1 is generatedwhen the prongs 1 and 2 move symmetrically with respect to one another,i.e., when the prongs 1 and 2 move in opposite directions, eithersynchronously away from one another or closer to each other, these twocases generating, respectively, positive and negative voltages. Thispolarity is chosen arbitrarily to simplify the discussion and could bechanged without loss of generality. The prongs 2 and 3 are wired togenerate a voltage V2 of opposite polarity, i.e., a positive voltage isgenerated when the prongs 2 and 3 move closer to each other. Apreamplifier (not shown) amplifies the difference between the voltages,V1-V2.

The acoustic wave generated by the relaxation of the gas between prongs1 and 2 excites predominantly the anti-symmetric mode (FIG. 10C) thatincreases or maximizes the difference V1-V2. Conversely, a commonexcitation of the prong pairs 1-2 and 2-3 excites predominantly thesymmetric mode (FIG. 10B), and thus reduces or minimizes the differenceV1-V2. The electrical pick-up, assuming the geometry of the device(i.e., wire geometry, length, or orientation, presence of potentialshielding, fork location, etc . . . ) is chosen to increase or maximizethe excitation of the symmetric mode by the electromagneticinterference, can thus be reduced or minimized.

In FIG. 11 we show a QEPAS setup 1100 using two forks 1101 and 1102wired in such a way as to subtract the electrical signals generated byeach fork. A differentiating pre-amplifier circuit 1103 can be used toperform that subtraction. The laser beam excites one fork 1101 and thesecond fork 1102 is positioned in such a way as to balance theelectromagnetic interference signal picked up by the two forks. Thesecond fork 1102 could be replaced by another component, such as a smallantenna. This QEPAS setup 1100 cancels out the common electromagneticpick-up driving both forks 1101 and 1102. Since the mechanical andelectrical properties of a piezoelectric tuning fork are coupled, theresonance frequencies of the forks 1101 and 1102 may be matched tobalance the pick-up signals excited in the two forks 1101 and 1102.

To account for potential differences in electromagnetic pick-up signalfrom the two forks 1101 and 1102 (or the fork and the antenna), afactory calibration can be realized, with the assumption that therelationship between the amplitude and phase of the two pick up signalsis constant in time.

In FIG. 12, we show a circuit 1200 for electromagnetic pick-up noisesubtraction. A switch 1210, realized here with transistors directlydefined on a semiconductor laser chip 1202 (e.g., indium phosphide inthe case of quantum cascade lasers), can allow for the alternating driveof a laser source 1220 and of a dummy load 1230 with electricalcharacteristics similar to the laser, located in close proximity to thelaser 1220, e.g., on the same chip 1202. It is expected that the twoloads (the laser 1220 and the dummy load 1230) and their mostly shareddrive circuits, generate similar electromagnetic radiation when drivenby current pulses. Two modulation schemes can then be realized:

(1) If f is the acoustic modulation frequency, the switch 1210 canalternate between driving the two loads at the same frequency f. Inother words, the driver delivers pulses at a 2f repetition frequency,alternatively to the laser source 1220 and to the dummy load 1230 (asingle pulse each per acoustic modulation period—or in a manner similarto the one described above, a short burst of pulses with total durationsmaller than the acoustic modulation period). The electromagneticpick-up signal is in this case modulated at 2f while the acoustic signalis modulated at f.

(2) The switch 1210 can allow for the drive of the laser source forseveral acoustic modulation periods, then drive the dummy load for asimilar time. The signal is digitized and the two portions are latersubtracted in digital domain.

QEPAS Integrated Implementations

We now present different device geometries to implement QEPASspectrometers in compact form factors. The different geometriespresented here integrate the semiconductor laser source and QEPASspectrophone in compact sensing platforms. The solutions aim atsimplifying the assembly process, for low-cost production. The basicelements of a QEPAS device are: (1) a semiconductor laser source; (2) atuning fork transducer; (3) one or several optical elements to image thelaser facet to a point between the fork prongs; and (4) electronicscircuitry to drive the laser and amplify and digitize the tuning forksignal. We show how several of these elements can be integrated in aneasy-to-assemble, low-cost platform.

In FIG. 13, we present a first device 1300 using a common patternedsubmount 1310 to mount a semiconductor laser chip 1320 (including alaser active region 1322 on a laser substrate 1324), an optical element(ball lens) 1330, and a tuning fork 1340. The optical element 1330images the laser facet to a point in between the tuning fork prongs. Asimple optical element such as a ball lens can be used. The submount1310 may be pre-patterned with structures that can be lithographicallyor otherwise defined. These features can include one or several of thefollowing: (1) a shallow stop ridge 1312 that the laser chip can beabutted to; (2) a dimple or v-shaped grooves 1314 to reference theposition of the optics; (3) a slit that the tuning fork can be insertedinto, or a pattern on the submount facet that the tuning fork can bereceived into. These alignment patterns can be etched or machined intothe submount 1310, or can be based on the deposition of metal pads(potentially lithographically defined) that the elements are solderedto. A die bonder or micropositioner may be used to assemble thedifferent elements in an automated fashion. The submount may be made ofa range of different materials (silicon, aluminum nitride, aluminumoxide, . . . ) that are usually employed as submount materials foroptical or electronic components. A common heat dissipation andtemperature control interface 1350 may be used, such as a thermoelectriccooler, a passive heat sink, a vapor chamber, a heater, etc. The laserchip 1320 may be mounted epi-up or epi-down, depending on the desiredthermal properties and heat dissipation constraints on the laser. Theoptical element 1330 may be refractive or reflective.

As shown in FIG. 14, an aperture in a mask 1442 (e.g., an iris) in closeproximity to the tuning fork 1340 can be used for spatial filtering ofthe beam. In particular, the wings of the beam can be filtered out toavoid having them hit the tuning fork prongs and generate a backgroundsignal. In all implementations discussed here, including the device 1300shown in FIG. 13, this feature can be added.

FIGS. 15A-15I show top views of QEPAS based on edge-emittingsemiconductor lasers. The laser epi layers are in the plane of theschematic. The tuning fork prongs are represented as two crossedrectangles. The architectures shown in FIGS. 15A-15I can also be usedfor resonant thermomechanical measurements by allowing the laser lightto hit the tuning fork instead of focusing the light between the tuningfork's prongs. This enables the tuning fork to act as a light intensitydetector that measures a reduction in transmission (intensity dip, or inother words a negative signal) in the presence of an analyte.

In FIG. 15A, a standard edge emitting semiconductor laser 1520 with astraight waveguide is free space coupled to an optical element 1530(refractive or reflective) that images the laser facet to a pointbetween the prongs of the tuning fork 1540.

In FIG. 15B, we show that a tapered section 1522 can be used to reducethe beam divergence in the plane of the schematic (‘in-plane’).

In FIG. 15C, we show that the tuning fork 1540 can be placed directly infront of the laser facet, without optical elements. The mitigationsdiscussed above can be used to reduce any background signal due toelectromagnetic interference or due to light hitting the fork prongs.Features on the semiconductor chip submount can be used to facilitatealignment of the laser waveguide and tuning fork. Such features mayinclude: defining a slit into the submount that the fork is insertedinto, defining a pattern (groove or solder pad) on the edge of thesubmount that can receive the fork in a particular position. The laserchip itself may be aligned precisely with the submount features (e.g.,solder pad) using a die bonding equipment.

In FIG. 15D, we show that the tapered section 1522, reducing thein-plane beam divergence, can be used to reduce risk of light hittingthe tuning fork prongs. The geometry of the tuning fork 1540 may be suchthat the space between the prongs is much higher than it is wide. Thevertical (out of plane) divergence of the beam is thus far less likelyto result in a background signal.

In FIG. 15E, we show how a focusing reflective optic 1532 can be used toimage the laser facet to a point between the tuning fork prongs.

In FIG. 15F, we show that a cylindrical lens 1534 can be used tocollimate the laser beam in the out-of-plane direction, while areduction of divergence in the plane of the schematic is achieved forexample with a tapered waveguide section 1522 in front of the laser.This tapered section can be active or passivated, as discussed below.

In FIG. 15G, we show that a cylindrical lens 1534 (rotated 90 degreesabout the optical axis from its orientation in FIG. 15F) can be used tocollimated the laser beam in the plane of the schematic. The geometry ofthe tuning fork 1540 is such that the spacing between the prongs isnarrow and high (several millimeters long). In other words, thedimension of the spacing between the prongs in the directionout-of-plane of the schematic is a few millimeters, compared totypically a few hundred micrometers for the prong spacing in the planeof the schematic. The divergence of the laser beam out of the schematicplane is thus less likely to cause background signal by hitting theprongs. It may thus be left uncollimated. This can simplify alignmentand the assembly process.

In the case of a quantum cascade laser, the substrate and claddingmaterials are typically InP while the active region is composed ofInGaAs and InAlAs. The index contrast between InGaAs/InAlAs and InPallows for vertical optical confinement (out-of-plane). In-planeconfinement is usually provided by etching a waveguide structure. Aninsulator and a material contact can further define lateral confinement.The optical losses are dominated by free carrier absorption. Ionimplantation can create traps for the free carriers to passivate thematerial, making it electrical insulating and optically low loss. Such aprocess can be used to passivate the tapered section 1522 and or theoptical relay section. An integrated focusing reflector (like the optic1532 in FIG. 15E) can be define by etching a groove in the lasermaterial, followed by deposition of a reflective metal layer.

In FIGS. 15H and 15I, we show that a section of passivated straightmaterial 1524 or passivated tapered material 1526 can be used toseparate the active material 1520 from the tuning fork, in case the fasttemperature rise of the active region 1520 creates acoustic wavespropagating in air from the facet of the laser. Such waves may be pickedup by the tuning fork 1540 and may result in a background signal. Ashort (10-500 μm) section of passive material may prevent the waveguidefacet (i.e., the area where the light is exiting the semiconductor chip)from experiencing significant temperature variations, and thus avoid orreduce the generation of acoustic waves. The tuning fork 1540 may thusbe placed in close proximity to the semiconductor chip edge with lessrisk of picking up a background signal from temperature variations ofthe laser material. The passivation can be realized as discussed above.

In FIGS. 16A and 16B, we show that the optical relay function achievedwith external optical elements in the systems shown in FIG. 15 (i.e.,the imaging of the laser facet onto a point located between the tuningfork prongs), can be integrated into the semiconductor material itself.The laser may use a tapered section 1522 as shown, although this is notrequired. The tapered section 1522 as well as the optical relay sectionsof the chip may be passivated and made transparent by ion implantationor selective area regrowth. The vertical confinement is provided by aslab waveguide geometry, for example, using the laser active material ascore, and the laser cladding material as cladding. The location of thefocus can be chosen to be near the semiconductor chip edge (from a fewmicrometers to a few millimeters) to facilitate assembly by bonding orotherwise mechanically registering the tuning fork to the semiconductorchip itself, or its submount.

In FIG. 16A, the optical relay is achieved by etching a curved (e.g.,parabolic or elliptical) reflecting groove 1630 that images the laserfacet onto a point between the tuning fork prongs.

In FIG. 16B, the optical relay is achieved by a waveguide lens 1632,realized, for example, by etching a pattern in the semiconductor lasertop cladding to vary spatially the effective refractive index of theguided mode. The pattern may be a subwavelength hole of groove pattern,or it may be a single large dimple defined by greyscale lithography.Alternatively, ion implantation can be used to tune spatially theeffective refractive index of the guided mode.

In FIGS. 17A-17C, we show that surface emission of the semiconductorlaser (e.g., quantum cascade laser) can be obtained by etching an angledfacet at one end of the laser waveguide 1722 a-1722 c, so as to reflectthe laser light towards the substrate or towards the top cladding.Openings in the metallization of either the bottom laser substrate 1724or the top cladding 1726 a-1726 c allow the light to escape to air. InFIG. 17A, the laser is more conveniently mounted epi-side up, whereasthe geometry shown in FIG. 17B is more amenable to flip-chip mounting ofthe semiconductor laser chip. The angle chosen may be 45 degrees, butother angles may be used, for example for ease of manufacturing by usingwet etching along crystalline planes. Since low laser feedback isexpected at this etched facet, the laser may include either an etchedreflecting structure (such as a deep groove) before the facet (as shownin FIG. 17C), or the laser would be a distributed feedback laser thatdoes not rely on facet reflection to build up the intracavity lightintensity.

In FIG. 18A a schematic of the laser 1800 is shown, with substrateemitting laser mounted epi-side down. The laser 1800 includes an activeregion 1822 sandwiched between a laser substrate 1824 and a lasercladding 1826, which are sandwiched in turn between a submount 1810 anda low-growth substrate. FIG. 18B shows a cross-section of the device1800, showing the InP:Fe regrowth of the buried hetero-structure 1801,the high mobility 2D electron gas used for back contact 1802 and themetal via 1803 connecting the back contact 1802 and the submount 1810.This illustrates how to route the two terminals of the laser diode tothe same side of the wafer, allowing contacting of both terminals with asingle step flip-chip bonding. This offers the advantage to allow forthe use of a low-doped (low electrical conductivity and low opticallosses) substrate, to avoid optical losses as the beam propagatesthrough the substrate.

In FIG. 19, we show an exploded view of an integrated QEPAS sensor 1900(subassembly) with laser source 1920, optical element 1930, aperture1942, and tuning fork 1940. These components can be assembled with awafer-bonding type process as shown in FIG. 20, whereby the differentfunctionalities are merged into a single subassembly by bonding waferswith two dimensional arrays of lasers 1920, lenslet arrays 1930,apertures 1942, and tuning forks 1940. After dicing the bonded waferstack (or a substack) into single discrete assemblies, these can be diebonded to a submount. Several subassemblies can be bonded to a singlesubmount 1910. The submount can be mounted onto an electronics board orother system components, for example, with ball bonding as shown in FIG.19.

In the device shown in FIG. 19, the laser 1920 is substrate-emitting(i.e., the laser light is directed towards the substrate with an angledfacet) and includes an active region 1922 between a cladding 1924 and alaser substrate 1926. In the next layer, a lenslet 1930 or other opticalcomponent compatible with low-profile wafer-scale manufacturing (e.g., ametamaterial flat lens) is used to image the laser facet onto a point inbetween the prongs of the tuning fork 1940. The next layer can be asilicon wafer with integrated drive circuitry and preamplifier for thetuning fork 1940. An array of apertures 1942 in this wafer allows thelight to pass through and reach the upper wafer containing the tuningfork 1940. This aperture 1942 can be used for spatial filtering of thelaser beam, to remove side lobes or ‘wings’, and prevent generation of abackground signal that would occur if some laser light was directlyhitting the prongs of the tuning fork. The last layer contains thetuning fork 1940 and its metalized contacts. An opening behind the forkallows the light to freely escape without being absorbed or scattered bythe tuning fork layer material. The optics 1930 and aperture 1942 layersare optional, depending on the level of integration desired.

In a possible assembly process, wafers containing the optics 1930 andlasers 1920 would be first aligned and bonded to one another. The waferscontaining the apertures 1942 and tuning forks 1940 would be similarlyaligned and bonded. Finally, the two pairs of bonded wafers would bealigned and bonded.

Alternatively, the optics wafer may be aligned and bonded to the laserwafer, then the aperture wafer to the bonded optics and laser wafers,then the tuning fork layer to the bonded aperture, optics, and laserwafers.

Metal vias through the different wafers can enable the routing of powerand electrical signals from one layer to its neighboring layers.Electrical connections between the sensor subassembly and the rest ofthe system may be done at the bottom of the stack (the tuning forklayer) by soldering to a metalized submount or using wire bonds betweenthe submounts and the top layer, or intermediate layers provided openingin the upper layers are defined prior to assembly.

As shown in FIG. 19, the sensor subassembly may be realized so as topartially enclose the tuning fork 1940 in a cavity. By selecting thematerials forming the walls of this cavity, and in particular by usingelectrically conductive materials (doped semiconductor, deposition of ametal layer on the walls), a Faraday cage may be realized to helpisolate the tuning fork 1940 from electromagnetic interferences.

The walls of the tuning fork cavity may also be coated with chemicalsfor selective absorption or adsorption of target analytes via weakreversible chemical bonds, thereby providing a concentration function. Athin film heater may be embedded in one of the layer (for example, theaperture layer) to trigger the release of the analytes for immediateanalysis by the sensor 1900. That desorption may be realized with asuccession of well-defined temperatures or with well-defined temperatureramps to allow for time-resolved desorption study. This may allowdifferent analytes to be measured at separate times, and thereby lowersthe risk of chemical interference in the measurement.

A heater may be embedded in a layer (for example, in the aperturelayer). For example, a thin electrical conductor may be defined on theaperture layer such that when current flows through the conductor, heatis deposited into the subassembly by Joule effect. Such heater may beused to remove contaminant that may have been deposited on the fork ofthe surrounding walls, potentially affecting the measurement.

System decontamination by heating the entire system or some subsectionof the system may be evaluated by monitoring the resonance curve of thetuning fork. To acquire such resonance curve, a sinusoidal voltage maybe applied to the tuning fork, and its response may be monitored. Ashift or change on amplitude of the resonance peak may indicatecontamination.

In FIG. 20, we show how different prefabricated wafers may be bondedtogether and then diced to yield sensor subassemblies, as discussedabove with respect to FIG. 19.

In FIGS. 21A-21D, we present a set of QEPAS sensors 2100 a-2100 d withsurface emitting semiconductor lasers 1920 that can be modulated usingany of the pulse modulation schemes disclosed herein. All schematicsshow the devices 2100 a-2100 d seen from the side (cross section alongthe length of the lasers). Each device 2100 includes a laser activeregion 2122 that is between a laser substrate 2124 and a laser cladding2126 and that emits a laser beam that is imaged between the prongs of atuning fork 2140. The tuning fork prongs 2140 are represented as twocrossed rectangles in most drawings.

In FIG. 21A, an angled facet (e.g., 45 degree) is defined at one end ofthe laser waveguide 2122 to direct the light towards the substrate 2124.The laser substrate 2124 may be thinned down to between 50 μm and 500 μmto reduce optical losses. Semi-insulating material, such as low-doped oriron doped InP, may be used in the case of quantum cascade lasers. Thelight diverges from the angled facet. At the bottom of the substrate2124, an optical element 2130 a may be integrated, for example by (1)bonding of a micro-optical element such as a small refractive lens; (2)etching of a lens into the substrate material; (3) etching of ametamaterial flat lens into the substrate material, formed for exampleby a series a thin grooves; (4) deposition and patterning of a material(dielectric or metal) to define a flat lens (Fresnel lens, metasurfacelens) onto the substrate bottom surface. This optical component 2130 aforms the image of the laser facet onto a small spot located a shortdistance beneath the substrate surface (from a few micrometers to a fewmillimeters). The tuning fork 2140 can be positioned at this spot, andpotentially integrated into a hollow submount 2110 as shown in FIG. 21A.The submount can have an opening in the beam path below the tuning forkto avoid light absorption and generation of acoustic waves.

In FIG. 21B, a reflective optical element 2130 b is defined on thebottom side of the substrate 2124 to form the image of the laser faceton the laser side of the wafer. The optical element 2130 b may bedefined using the methods described above. An anti-reflection coating(not shown) may be used to reduce reflection at the top substratesurface where the beam emerges into the air. This geometry may alloweasier integration and alignment of the tuning fork on the laser-side ofthe chip. The tuning fork 2140 may be fabricated out of the samesemiconductor material as the laser itself (e.g., InP) and thusfabricated with similar microfabrication techniques as the laser itself(photolithography, wet/dry etching, etc . . . ).

In FIG. 21C, the bottom substrate surface is coated by a reflectivematerial 2132 but is otherwise flat. Imaging of the laser facet to apoint a short distance above the wafer surface is achieved with anoptical element 2130 c integrated (following any of the methodsdescribed above) onto the top substrate surface. As in FIG. 21B, thetuning fork 2140 may be fabricated directly from the laser material ormade from other materials including quartz aligned and assembled a shortdistance (few micrometers to a few millimeters) above the semiconductorlaser chip surface.

In FIG. 21D, the angled facet is defined in such a way as to direct thelight towards the top cladding. An external optical element 2130 d maybe aligned above the semiconductor laser chip to image the laser facetto a point a short distance above the chip surface, where a tuning fork2140 may be aligned and attached to the chip.

CONCLUSION

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific inventive embodiments described herein. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that inventive embodiments may be practicedotherwise than as specifically described and claimed. Inventiveembodiments of the present disclosure are directed to each individualfeature, system, article, material, kit, and/or method described herein.In addition, any combination of two or more such features, systems,articles, materials, kits, and/or methods, if such features, systems,articles, materials, kits, and/or methods are not mutually inconsistent,is included within the inventive scope of the present disclosure.

Also, various inventive concepts may be embodied as one or more methods,of which an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein, unless clearlyindicated to the contrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein, should be understood to mean“either or both” of the elements so conjoined, i.e., elements that areconjunctively present in some cases and disjunctively present in othercases. Multiple elements listed with “and/or” should be construed in thesame fashion, i.e., “one or more” of the elements so conjoined. Otherelements may optionally be present other than the elements specificallyidentified by the “and/or” clause, whether related or unrelated to thoseelements specifically identified. Thus, as a non-limiting example, areference to “A and/or B”, when used in conjunction with open-endedlanguage such as “comprising” can refer, in one embodiment, to A only(optionally including elements other than B); in another embodiment, toB only (optionally including elements other than A); in yet anotherembodiment, to both A and B (optionally including other elements); etc.

As used herein, “or” should be understood to have the same meaning as“and/or” as defined above. For example, when separating items in a list,“or” or “and/or” shall be interpreted as being inclusive, i.e., theinclusion of at least one, but also including more than one, of a numberor list of elements, and, optionally, additional unlisted items. Onlyterms clearly indicated to the contrary, such as “only one of” or“exactly one of,” or, when used in any claims, “consisting of,” willrefer to the inclusion of exactly one element of a number or list ofelements. In general, the term “or” as used herein shall only beinterpreted as indicating exclusive alternatives (i.e., “one or theother but not both”) when preceded by terms of exclusivity, such as“either,” “one of,” “only one of,” or “exactly one of” “Consistingessentially of,” when used in any claims, shall have its ordinarymeaning as used in the field of patent law.

As used herein, the phrase “at least one,” in reference to a list of oneor more elements, should be understood to mean at least one elementselected from any one or more of the elements in the list of elements,but not necessarily including at least one of each and every elementspecifically listed within the list of elements and not excluding anycombinations of elements in the list of elements. This definition alsoallows that elements may optionally be present other than the elementsspecifically identified within the list of elements to which the phrase“at least one” refers, whether related or unrelated to those elementsspecifically identified. Thus, as a non-limiting example, “at least oneof A and B” (or, equivalently, “at least one of A or B,” or,equivalently “at least one of A and/or B”) can refer, in one embodiment,to at least one, optionally including more than one, A, with no Bpresent (and optionally including elements other than B); in anotherembodiment, to at least one, optionally including more than one, B, withno A present (and optionally including elements other than A); in yetanother embodiment, to at least one, optionally including more than one,A, and at least one, optionally including more than one, B (andoptionally including other elements); etc.

As used herein, all transitional phrases such as “comprising,”“including,” “carrying,” “having,” “containing,” “involving,” “holding,”“composed of,” and the like are to be understood to be open-ended, i.e.,to mean including but not limited to. Only the transitional phrases“consisting of” and “consisting essentially of” shall be closed orsemi-closed transitional phrases, respectively, as set forth in theUnited States Patent Office Manual of Patent Examining Procedures,Section 2111.03.

1. A method of making a spectroscopic measurement of a sample, themethod comprising: emitting a sequence of pulses from a single-modelaser; illuminating the sample with the sequence of pulses; anddetecting radiation transmitted, reflected, and/or scattered by thesample in response to the sequence of pulses with a detection systemhaving a low-pass cutoff frequency less than a pulse repetitionfrequency of the sequence of pulses.
 2. A method of making aspectroscopic measurement of a sample, the method comprising: modulatinga single-mode laser with a repetitive pulse sequence so as to cause thelaser to emit a periodically pulsed laser beam, each period of therepetitive pulse sequence comprising a plurality of pulses; illuminatingthe sample with the periodically pulsed laser beam; generating anelectrical signal representing radiation reflected, scattered,transmitted, and/or emitted by the sample in response to theperiodically pulsed laser beam; and bandpass filtering the electricalsignal at a band centered on the pulse repetition frequency.